heat flux and heat transfer coefficient

Hi guys,

I was going through heat transfer tutorial and have a slight confusion with the following:

What is the significance of using both heat flux and convective heat coefficient here? And why heat flux is set zero? Do I need to define both of them? Or, can I use either heat flux or convective heat transfer coefficient?

The link is:

https://2022.help.altair.com/2022.1/hwcfdsolvers/acusolve/topics/tutorials/acu/acu_3110_intro_cfd_r.htm#acu_3110_intro_cfd_r

Thanks!